This invention relates to an analog-to-digital converter and, more particularly, to such an A/D converter that is relatively inexpensive to construct, operates quickly to digitize transient signals and provides an accurate multi-bit (for example, a 12-bit) digital signal.
So-called parallel-serial-parallel A/D converters are known in which two separate stages are used to convert an input analog signal to a plural-bit digital signal. In one example of such parallel-serial-parallel converters, the analog signal is sampled and the sample is converted to, for example, a 4-bit coarse digital representation. This coarse digital representation is re-converted to a coarse analog signal, and the difference between the input analog signal and this coarse analog signal is converted to, for example, a 4-bit fine digital representation. The four bits which comprise the coarse digital signal constitute the bits of greater significance of an 8-bit digital signal, and the bits which comprise the fine digital signal constitute the four bits of lesser significance. However, the fact that the fine bits are produced by means of an A/D converter that operates on the sensed (and changing) difference between the original analog signal and the coarse analog representation thereof may introduce errors into the accuracy of the fine digital representation. Also, the coarse and fine A/D converters may operate too slowly for the successful digital conversion of an input transient signal that varies at high frequencies.
Another example of a previously proposed A/D converter utilizes a ramp waveform signal of, for example, increasing magnitude, which is compared to a sample of the input analog signal. A clock pulse counter is turned on concurrently with the ramp waveform and is turned off when that waveform reaches the sample level. The resultant count of the clock pulse counter is a digital representation of the analog sample. Unfortunately, the time required for this counting is sufficiently long that this A/D converter is not desirable for digitizing rapidly changing transient signals.
A variation of the last-mentioned A/D converter employs a negative ramp waveform to discharge a capacitor across which the analog sample is stored; and the clock pulse counter is turned off when the capacitor is discharged to zero. However, this A/D converter suffers from the same disadvantages mentioned above.